I'll start with the downs.
The game could definitely use more tracks, only eight cups doesn't seem like enough for such a seemingly big game. Also, they most definitely did over-use the "baby" characters. Also, I don't like some of the alterations to the past tracks.
Now with the good
The levels are beautiful and well-designed, the character roster is for the most part awesome, the Wheel is a great feature to include with this game, and the most important reason as to why it's good.... it's freakin' Mario Kart.